Languages have standards as well as variations which lead to changes over time.

Accent

Everyone who speaks a language has an accent. You and someone else can use the same words with the same intended meanings but when you say them you may pronounce them differently. That difference is your accent. An accent is how words are pronounced.

We frequently think of accents based on geography (the Boston accent, the New York City accent, etc) but going to these places you realize not everyone in these regions has the same accent. This is because accents are also influenced by social class, ethnicity, and other factors.

Dialect

While an accent is narrowly focused on pronunciation, a dialect is a broader group of linguistic variations. A dialect accounts for variations in word meaning, grammar, as well as accent – accent is just one of several parts that make up a dialect. As such, just like accents, dialects are influenced by geography, social class, ethnicity, etc.

A good example of dialects at the national level are the different versions of the English language between Australasia, the British Isles, and North America. These three areas are using the same language but they have enough difference between them to make up different dialects. Then within each area are smaller regional and sub-dialects.

Generally speaking if we compare the English dialect to the American dialect we get different accents but also word variations. A lift is an elevator, a boot is a car trunk, a jumper is a sweater, etc. Spelling differences however, like colour vs color, is a difference of language.

Others

So what about Jargon? Patois? Pidgin? Slang?

When two groups of people need to communicate, and there isn’t a shared language between them, they sometimes create a simplified hybrid language. This new language borrows from the native languages of both groups. This new developing language is a pidgin.

When a pidgin language stabilizes and becomes the primary language spoken by a community it becomes a creole. Creole is a contact language that is mix of other established languages and is the primary language of a community (as opposed to a pidgin which is spoken only to facilitate communication by two groups who ordinarily speak other languages).

Patois does not have a standardized meaning and and as such can be a pidgin, a creole, a dialect, etc.

Jargon and slang are both vocabulary based. Jargon is technical terminology related to an area of study or occupation. Slang is common informal vocabulary used by people of a certain in-group.

Prescriptive or Descriptive

All of these linguistic variations show that languages change. No language remains stationary, frozen to some moment in time. For example, attend a performance of Shakespeare and you are reminded that the English language of today is far removed from that of the 16th century. The language you learned as a child is not the same language as today.

There are two approaches to language: prescriptive and descriptive. Prescriptive is rule based, it sets forth how words & language should be used. Descriptive describes how things are actually being used, documenting the changing linguistic reality of the times.

Languages need a mix of prescriptive and descriptive approaches. To only describe language becomes a free for all where everyone does whatever they want. To only prescribe rules means a language never changes to the needs of the people speaking it. Linguistic shift is the process that languages change over time as societies change over time.

Added info: a brogue is another linguistic term that lacks clear definition (sometimes it’s an accent, others it’s a dialect). Typically though it is another way of saying Irish accent.